This invention relates to the operation of electric powered furnaces, particularly electroslag remelting furnaces (ESR) and to a method and apparatus of continuously controlling the atmosphere within such furnaces to result in improved chemistry control and improved cleanliness of the final produced metal.
Some electroslag remelted ingots contain reactive elements which are easily oxidized, and therefore present serious problems in both chemical composition control and cleanliness of the metal. The atmosphere above the slag bath in an electroslag furnace has a dramatic influence on both the chemistry and cleanliness of the metal of the remelted ingot. Both the oxygen contained in the air and the moisture content of the air normally present in the furnace are significant factors. The oxygen combines with reactive elements to form oxides. The oxides are retained in the slag bath thereby depleting the metal of that specific element. Elements removed by this mechanism often cause the base metal to fall below desired specifications for these elements. Moreover, the oxidization process is not uniform throughout the finally produced ingot. Particularly, reactive elements are depleted more rapidly at the start of the melting process. The oxidization rate is gradually reduced as melting proceeds until it abates completely when the content of the reactive element in the metal is in equilibrium with its oxide species in the slag. Consequently, in many cases it is not possible to hold critical elements within specification nor to maintain them uniformly from top to bottom of the resultant ingot. In addition the composition gradient between top and bottom of the melt often presents difficulties in determining heat treatment times and temperatures to develop specific mechanical properties.
A basic consideration of the control of the furnace atmosphere above the slag bath is &:he need to develop a means and method to preclude the presence of oxygen from the atmosphere. In order to determine the effectiveness of any techniques devised to accomplish this end, it is necessary to find an effective method and means to measure the oxygen content above the slag bath. Analytical equipment is available to accurately determine the oxygen level. A problem exists, however, in the sampling of the atmosphere above the slag bath. Oxygen analyzers draw a gas sample from the furnace atmosphere which contains solid particles evolving from the slag and very quickly plug the sample tube. Consequently, in the past, atmosphere samples could be obtained only during the early portion of the heat.